Regarding the slowness of your Macbook download and run Etrecheck. Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here (and recommended by Apple Support personnel) in the ASC to help identify the more obvious culprits that can adversely affect a Mac's performance.
You can delete the old iPhoto library as long as you're sure you want to stay with the new Photos. The two libraries don't take up twice the space on the hard drive as described in this Apple document: Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support so there's very little space to be regained by deleting it.
Photos is new and currently is missing a lot of iPhoto's features and capabilities. You might want to keep both libraries working until you're sure Photos will do what you want it to do. This user tip describes the differences between the two apps: Photos 2.0 vs iPhoto 9.6.1: Features and Capabilities
Tell Apple what missing features you'd like restored or new features added in Photos via Apple-Photos Feedback.
The feedback is working as can be seen from the following paragraph in this Apple webpage: Apple OS X El Capitan.
Photos has been fine-tuned to make it even easier to manage your library. Now you can add a location to a single image or to an entire Moment. Naming your favorite people in Faces is faster with a streamlined workflow. You can also sort your albums — and the contents inside them — by date, title, and more.
